A problem with diesel engine exhaust gas includes nitrogen oxide (NOx) and PM. Above all, PM is a fine particulate mainly comprising carbon, and as a general method for removing it, there is mentioned a method for trapping PM by disposing a diesel particulate filter (DPF) that comprises a porous ceramic material, in an exhaust gas duct. The trapped PM is intermittently or continuously processed for combustion, and the DPF is regenerated to its state before PM trapping.
For the DPF regeneration treatment, generally employed are a method of combusting PM by forced heating from the outside with an electric heater, a burner or the like; a method comprising disposing an oxidation catalyst on the side nearer to engine than DPF and converting NO in exhaust gas into NO2 with the oxidation catalyst to thereby burn PM by the oxidation force of NO2; etc. However, use of an electric heater or a burner requires installation of an external power source for it, further requiring an additional mechanism or the like for maintaining and operating them; and therefore, the system itself shall be complicated. In addition, the oxidation catalyst has some problems to be solved in that the exhaust gas temperature is not so high to such a degree that the catalyst could sufficiently exhibit its catalytic activity, and the exhaust gas could not contain NO necessary for PM combustion unless it is not under a certain driving condition. As a more preferred regeneration treatment method for DPF, a method is being investigated, which comprises making DPF itself carry a catalyst to thereby lower the PM combustion starting temperature by the catalytic action, followed by PM combustion. As an ultimate object, a method of continuous combustion at an exhaust gas temperature is considered the best.
At present, as an oxidation catalyst for combustion and removal of PM trapped by DPF (PM combustion catalyst), used is a catalyst metal Pt supported by alumina or the like having a high specific surface area. However, on an exhaust gas temperature level, the catalytic effect action of Pt for PM combustion is low, and therefore, continuous PM combustion with the heat of exhaust gas is considered difficult. In addition, another problem is that Pt is expensive and causes cost increase.
Still another assumed problem with a PM combustion catalyst is that the catalyst temperature may rapidly increase owing to the heat generation in PM combustion. Accordingly, expected is development of catalyst substances of which the catalyst capability in exposure to a thermal history at high temperatures degrades as little as possible (that is, those resistant to thermal deterioration). Patent References 1 to 3 disclose, as an oxidation catalyst based with a ceria composite oxide not containing a noble metal element such as Pt or the like, a mixture containing Ce and Bi alone, or further containing a transition metal element. The oxide solid solution comprising Ce and Bi alone, or comprising Ce, Bi and a transition metal element releases Bi when exposed to a high temperature of around 800° C. for a long period of time, since the melting point of the element Bi is low, and therefore, the catalyst activity lowers. Accordingly, further improvement of the catalyst in point of the heat resistance thereof is desired.                Patent Reference 1: JP-A 6-211525        Patent Reference 2: JP-A 2003-238159        Patent Reference 3: JP-A 2006-224032        